1. Field of the Invention
The present invention relates to a stabilizing circuit for preventing a transistor from oscillating and an amplifier comprising the same.
2. Description of the Prior Art
In recent years, radio waves of an extremely large number of frequencies are required for communication following rapid development of mobile communication, and the frequencies of radio waves employed for mobile communication are now shifting to the microwave band. An amplifier which is applied to a portable machine for such mobile communication is provided with a stabilizing circuit for preventing a field-effect transistor (hereinafter referred to as an FET) from oscillating.
FIG. 13 is a circuit diagram showing an exemplary conventional stabilizing circuit. The stabilizing circuit shown in FIG. 13, which includes resistors R1, R2 and Ra and a capacitor Ca, serves also as gate bias means.
The resistors Ra and R1 are connected between the gate of an FET 100 and a ground terminal, and the resistor R2 is connected between the source of the FET 100 and a power supply terminal NG receiving a power supply voltage Vg. The node between the resistors Ra and R1 is connected to a ground terminal through the capacitor Ca. The resistors Ra, R1 and R2 form a gate bias circuit for applying a gate bias to the gate of the FET 100.
In the stabilizing circuit shown in FIG. 13, part of an input signal flows to the resistor Ra and the capacitor Ca. Thus, the resistor Ra causes circuit loss, to prevent the FET 100 from oscillating over all frequency domains.
FIG. 14 is a circuit diagram showing another exemplary conventional stabilizing circuit. The stabilizing circuit shown in FIG. 14 is formed by a feedback circuit provided with a capacitor Cf and a resistor Rf.
The resistor Rf and the capacitor Cf are connected in series between the drain and the gate of an FET 100. Resistors R1 and R2 form a gate bias circuit for applying a gate bias to the gate of the FET 100.
The stabilizing circuit feeds back an output signal which is outputted from the drain of the FET 100 to the gate in opposite phase. Thus, the FET 100 is prevented from oscillating in a high-frequency domain due to the negative feedback effect.
The stabilizing circuit shown in FIG. 13 prevents the FET 100 from oscillating by causing circuit loss. If the resistance value of the resistor Ra is larger than the impedance of the FET 100 in this case, a current hardly flows to the resistor Ra. Therefore, the resistance value of the resistor Ra must be reduced to some extent. Further, the capacitance value of the capacitor Ca must be relatively increased, for reducing its impedance.
In order to stabilize an FET having a large reflection coefficient (approximate to 1) and a high gain, for example, the resistance value of the resistor Ra must be reduced to not more than about 20 .OMEGA., and the capacitance value of the capacitor Ca must be increased to about 100 pF. A large occupied area is required for preparing the resistor Ra having a small resistance value, while a large occupied area is also required for preparing the capacitor Ca having a large capacitance value. Consequently, the chip area is increased.
In the stabilizing circuit shown in FIG. 13, the K value (stability factor) can be increased beyond 1 over the frequency domain of 0 to several GHz. Absolute stability is attained if the K value is larger than 1. However, stabilization of the FET 100 is reciprocal to the gain. When stabilizing the FET 100 over a wide frequency domain in case of preparing an amplifier for amplifying signals of a specific frequency domain, therefore, the gain in the specific frequency domain is disadvantageously reduced. While the FET 100 has no possibility of oscillation if sufficiently stabilized, no sufficient gain is attained at a desired frequency in this case.
On the other hand, the stabilizing circuit shown in FIG. 14 can stabilize the FET 100 in a high-frequency domain, while stabilization of the FET 100 may not necessarily be attained in a low-frequency domain. An FET requires negative resistance for oscillation. In general, this negative resistance is readily generated on a low-frequency side in case of employing a common-source FET (refer to "the Basis of MMIC Oscillator Design" by Takashi Ohira, NWE Microwave Workshop Digest, 1996, pp. 438-447). Further, the common-source FET is generally instable with a small K value on a low-frequency side. In order to prevent the FET from oscillating, therefore, it is important to attain stabilization in the low-frequency domain. The stabilizing circuit shown in FIG. 14, which can not necessarily prevent the FET 100 from oscillating in the low-frequency domain, is incapable of sufficiently stabilizing the FET 100.
Further, application of the feedback circuit to a power amplifier or the like, which requires consideration of a nonlinear operation, may result in subharmonic oscillation (refer to "Microwave Nonlinear Circuit Technique" by Kazuhiko Honjo, NWE95 Microwave Workshop Digest, 1995, pp. 65-74). Thus, a countermeasure for suppressing the subharmonic oscillation is required.